Phase locked loops (PLL) are negative feedback loops which are well known from the state of the art.
A PLL comprises a voltage controlled oscillator (VCO), which generates the output signal of the PLL. This output signal can be used for example as local oscillator signal for a receiver mixer of a receiver chain or a transmitter mixer of a transmitter chain in a cellular phone. The VCO is driven by a loop-filter, which determines the loop characteristics of the PLL, e.g. the settling time and the loop stability. The response of the loop-filter has therefore to be very accurate.
In order to reduce the number of external or discrete components, it is further desirable to use an integrated loop-filter in a PLL. With an integrated loop-filter, also the probability of a disruptive coupling is reduced. The values of integrated components, however, vary much more than the values of external components which are more accurate due to process variations or environmental influences. External Negative Positive Zero (NPO) capacitors, for example, have a very stable value over a wide temperature range, usually between −25° C. and +85° C.
Therefore, conventional PLLs generally comprise accurate external components for the loop-filter. When an integrated loop-filter is used nevertheless, a complicated calibration procedure is employed.